About pointers to a double derived class

[espoice]

Hello forum,

I have been struggling with this problem since a while.
Basically I have a FunzioneBase.h that works like this:

#include "Vettore.h"      //Vettore.h is acually an header file of a class which create some kind of container of numbers of selected dimension.
//I need this class instead of a <vector> for other purposes of the program, but the problem is not concerning this section.

class FunzioneBase {

	public:
		virtual double Eval(double x) const =0;    //line n°9, remeber this line, we'll get to that later
};

class sfera: public FunzioneBase, public Vettore {     //we see that sfera class is derived from two different base classes. 
//This is line n° 47, remeber this line because it wil be inside the compilation error

	public:

		sfera(int dimensions);    
		~sfera();

		virtual double Eval(double radius);

	protected:

		double _dimensions;
};

and its implementation:

sfera::sfera(int dimensions): Vettore(dimensions) { _dimensions = dimensions;}     //constructor which assigns a position vector in space of requested dimension 

sfera::~sfera() {}


double sfera::Eval(double radius){        
  //doing stuff.. I left out "const" because I need to modify components of "Vettore" inside this function, otherwise it will give a compilation error.
 //I'll post the code anyway, maybe it will give you more infos to help me solve the problem:

Random k(3);
	double d = 0;

		for(int i = 0; i < m_N; i++)	//m_N is the dimension of declared Vettore		

			SetComponent(i, sqrt(2*pow(radius*k.LCG(), 2) ); 	//SetComponent, GetComponent are Vettore.h methods, which intuitively you can imagine their purposes.
//Random k(3) and LCG() basically let me generate a random number between 0 and 1. 

	double expected = pow(radius, _dimensions)*(pow(M_PI, _dimensions/2))/(tgamma(_dimensions/2 + 1));

		for(int i = 0; i < m_N; i++)

			d += pow(GetComponent(i), 2);
		

	d = sqrt(d);

	double observed = pow(d, _dimensions)*(pow(M_PI, _dimensions/2))/(tgamma(_dimensions/2 + 1));

		if(observed <= expected)

			return 1;

		else

			return 0;

}

The problem occurs when I need to define a ponter to sfera, which happens in another .h file, "Integral.h", whose constructor works like:

class Integral {
	public:
		Integral(double a, double b, FunzioneBase* function);

        //metohds..

        private:

             FunzioneBase* _integrand;   

};

//Integral.h constructor is:

Integral::Integral(double a, double b, FunzioneBase* function) {

	_integrand = function;
        /* _a = min(a,b);    						
	_b = max(a,b);
	if (a > b) _sign = -1;
	else _sign = 1;  */  //we don't need those right now
}

I acutally need a pointer to FunzioneBase because I need that virtual Eval function declared in FunzioneBase.h in order to evaluate the integral of a "sphere", but it seems that isn't enough to let the Integral.h method workf, which is:

double Integral::HitOrMiss2(int nstep) {
		

		double area = 8;

		int n = 0;
		int passaggi = 0;

			do {	
	
					if( _integrand->Eval(1) == 1)
					
						n++;


				passaggi++;
						

			} while( passaggi < nstep);


 return area*n/nstep;

}

I ended up getting this error:

main.c:13 error: cannot allocate an object of abstract type 'sfera'
FunzioneBase.h:47: note: because the following virtual functions are pure within 'sfera':
FunzioneBase.h:9: note: virtual double FunzioneBase::Eval(double) const

I delcared this ponter in main.c as:

int main(){

       FunzioneBase* function = new sfera(3);   //line n° 13


	Integral* p = new Integral(0, 2, function);  
	
	cout<<p->HitOrMiss2(10000)<<endl;

return 0;

}

I think the problem is related to that pointing to "sfera" class. I wondered what would happen if I had wrote Vettore* foo = new sfera(3). Which is the correct syntax in order to avoid that error? Hope you guys can help me, it would be really appreciated.

[keskiverto]

How many functions do you see here:

virtual double Eval(double radius);
virtual double Eval(double radius) const;

[espoice]

Hi keskiverto, thanks for the reply.
That was the problem.
What if I write another "virtual double Eval(double radius)" function inside public FunzioneBase class without "const" restriction?

I need "virtual double Eval(double radius) const" with that "const" for other purposes I did not mention in the code I posted(other "funcition" classes which required "const" ). Will it work anyway?
Thanks again!

[ne555]

once you make `sfera' a concrete class, to create a `sfera' do sfera foo(3);

> I need to modify components of "Vettore" inside this function
¿why? you are just setting random numbers, ¿do you care about that state at all?


ps, fix your indentation.

[espoice]

Hello ne555, thanks for giving me your opinion.
Actually I'm applying this method for evaluating the volume of a sphere: https://en.wikipedia.org/wiki/Monte_Carlo_integration which requires random numbers.
The focus of the program is to verify the evolution of the S(n)/R(n) ratio, where S(n) is the volume of the n-dimensional sphere of radius 1 and R(n) is the volume of the n-dimensional "cube" (which Side is equal to 2) circumscribed to the sphere, using Monte Carlo Integration method. I'm trying to write it 3-dimensional.
I'm still working to get it run correctly, anyway I need those random numbers.


So the difference between declaring:
FunzioneBase* foo = new sfera(3);
and
Vettore* foo = new sfera(3);

what concerns?

[ne555]

> I'm still working to get it run correctly, anyway I need those random numbers.
my point is that you don't need them as the state of your object.
you may create a local `Vettore', or simply sum the numbers directly.

double d = 0;
for(int i=0; i<this->_dimensions; ++i)
   d += 2*pow(radius*k.LCG(), 2);
d = sqrt(d);

> So the difference between declaring:
¿why are you using new?
http://www.cplusplus.com/forum/general/138037/#msg731921